The field of the invention is automatic mobile telephone systems.
Automatic mobile telephone systems comprise a base stations and one or more mobile units each of which is provided with a radio receiver and a transmitter in addition to the necessary telephone equipment for ringing, connecting, disconnecting, etc.
In any one area of operation there are a limited number of channels available. Accordingly, it is essential that the channels be used effectively and efficiently in order that the maximum number of telephone conversations between parties can be carried on. For a mobile unit to have its receiver (handset) off hook when the unit is not actually talking to some other parties either mobile or otherwise, is essentially intolerable. Such a mobile unit is termed a wild mobile because its transmitter is sending the carrier frequency of that channel when the mobile unit is not actually talking to another party. That channel, or frequency, is being denied to some other mobile subscriber.
It is known in the art to shut off such wild mobile transmitters by having the base station sense, at intervals, when a mobile unit is wild and sending a disable, or shut off, signal. In the prior art, the disable signal has comprised an alternating sequence of 2,000 Hz and 1,800 Hz tones for a long period of time, for example, 30 seconds. The timing means sensed the length of time that the alternating tones, which may be the idle and seize tones, were received, and in response to the length of time, turned off or disabled the mobiles transmitter. Such a circuit while functioning reasonably well is subject to error because the timing circuit is subject to variations of its own. And in addition other tolerances in the apparatus make for a variable time, such for example, as temperature variations, voltage variations, humidity variations and aging characteristics of electronic components. The length of time necessary to be certain that the mobile is appropriately disabled ties up the channel for a needlessly long period of time.
Accordingly it is an object of the invention to provide a wild mobile disabling circuit which obviates the disadvantages of the prior art schemes.
It is a further object of the invention to provide an improved wild mobile disabling circuit of the nature indicated which shuts down the transmitter precisely, effectively and without being dependent upon the other variables in the system.
It is a further object of the invention to provide an improved wild mobile disabling circuit which utilizes faster signalling and may function to shut down the wild mobile in as short a period of time as the base station which sends the disable signal is able to send it. Thus there is no tieing up of a channel beyond what is necessary to shut down the wild mobiles transmitter.
According to the invention the improved wild mobile disabling circuit has no tolerance problems, is more reliable than known systems and has a faster shut down capability.